Permanent waving is a process whereby a reducing agent is applied to mandrel-wound hair to open the disulfide linkages of the hair structure which are formed by the amino acid cystine. The protein chains flow under tension to assume the imparted shape. After rinsing, an oxidizing agent is then applied to re-establish or close the disulfide linkages which, in effect, hardens the protein structure to lock it into the new position.
Hair can be relaxed or straightened by a converse process whereby the hair is chemically acted upon when the hair is straight or at a curl having a radius of curvature greater than that existing in the hair. To this end, the hair can be held straight or wound on rods or mandrels of large diameter.
An essential element of a hair straightening composition is sodium hydroxide or a reducing agent. Among the reducing agents there may be mentioned thioglycolic acid, salts, and esters thereof; thiolactic acid and salts thereof; sulfide salts; bisulfite salts, cysteine, and the like. Aside from sodium hydroxide, the bulk of the compositions for straightening hair are based on thioglycolic acid, salts, or esters thereof, or the bisulfite salts.
The compositions used for relaxing or straightening hair are normally provided as a gel or a cream which serves to mechanically hold the hair in a straightened state during chemical attack. Sodium hydroxide relaxers are generally cream compositions at concentrations up to 3%, preferably about 2 to about 3%, NaOH. Sodium hydroxide attacks the disulfide bond to produce lanthionine which now crosslinks the hair in its straightened form. The generalized reaction is: ##STR1## Following straightening, an acid rinse is usually applied to neutralize excess alkali.
The hair may be straightened with thioglycolates under acid conditions. To this end, citric, lactic, phosphoric, and weak carboxylic acids are used as common acidifying agents. Bisulfite straighteners are also acid, namely at a pH from about 5.5 to 6.9, and are applied at room temperature.
As with sodium hydroxide compositions, contact with the reducing agent may range from 10 minutes or less to 50 minutes or more. After an appropriate time of contact, the reducing agent is rinsed from the hair, and an oxidizing agent is applied to close the disulfide bonds and set the straightened hair. Excess oxidizing agent is then rinsed from the hair, and the hair is dried. The most common oxidizing agents are hydrogen peroxide and bromate salts. Peroxides are applied over a pH range from 2.5 to about 4.0 and bromates from a pH of about 5.0 to about 8.0. Application is at ambient or elevated temperatures.
In general, sodium hydroxide is the most effective straightener but is also the most damaging to hair and most likely to irritate the skin. Thioglycolate and alkali bisulfite straighteners do not produce the same degree of straightening but relax the curl from a tighter to a looser wave pattern.
All forms of hair straightening suffer by reversion. As the straightened or relaxed hair is subjected to shampooing, heat or high humidity, it reverts to some degree back to its curly form.